1. Field of the Invention
The present invention relates generally to the structure and use of radio frequency electrosurgical probes for the treatment of tissue. More particularly, the present invention relates to an electrosurgical probe having multiple wire electrodes which are deployed in an array to treat a volume of tissue, particularly for tumor treatment.
2. Background of the Invention
The treatment of bodily tissue by using thermal energy to destroy it is useful for various therapeutic procedures. Thermal energy can be imparted to tissue using radio frequency electrical energy, microwave or lightwave electromagnetic energy, ultrasonic vibrational energy, or thermal conduction.
Radiofrequency ablation (RFA) is becoming a popular medical alternative to treat patients with tissue anomalies who were previously not candidates for surgery. For example, RFA is commonly used to treat liver anomalies and many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney and lung. RFA treatment involves the destruction of undesirable cells by generating heat through agitation caused by the application of alternating electrical current (radiofrequency energy) through the tissue.
Various RF ablation devices have been designed to perform this treatment. See, for example, U.S. Pat. No. 5,855,576, which describes an ablation apparatus that includes a plurality of wires connected through a catheter. Each of the wires includes a proximal end that is connected to a generator, and a distal end projecting from a distal end of the catheter. The wires are arranged in an array with the distal ends located generally radially and uniformly spaced apart from the catheter distal end. The wire ends act as electrodes that may be energized in a monopolar or bipolar fashion to heat and necrose tissue within a precisely defined volumetric region of target tissue. The current can flow between closely spaced energized wire electrodes or between an energized wire electrode and a larger, common electrode located remotely from the tissue to be heated. In order to assure that the target tissue is adequately treated and limit damage to adjacent healthy tissues, it is desirable that the array formed by the wires within the tissue be precisely and uniformly defined. In particular, it is generally desirable that the independent wires be evenly and symmetrically spaced-apart so that heat is generated uniformly within the desired target tissue volume. The ablation device may be used either in an open surgical setting, in laparoscopic (small incision) procedures, or in percutaneous (through the skin) interventions.
During ablation of tissue, the maximum heating often occurs in the local tissue, immediately adjacent the emitting electrodes. In general, the level of tissue heating is proportional to the square of the electrical current density, and the electrical current density in tissue generally falls as the square of the distance from the electrode. Therefore, the heating in tissue generally falls as the fourth power of distance from the electrode and the resulting tissue temperature therefore decreases rapidly as the distance from the electrode increases. This causes a lesion to first form along the electrodes, and then between the electrodes.
For example, FIGS. 1A to 1D show how a desired thermal lesion is created using the above described ablation apparatus. Using conventional imaging methods such as ultrasound, an array 2 of wires 4 is positioned strategically within the targeted area of tissue and energized with electrical current. Initially, a thermal lesion 6 begins to form at the tips of the wires 4 (FIG. 1A). As the ablation process continues, the thermal lesion 6 expands along the wires 4 back toward the center of the array 2, as indicated by the directional arrow 7 (FIG. 1B). Next, the thermal lesion 6 expands outward and between the wires 4, as indicated by the directional arrow 8 (FIG. 1C), until the desired thermal lesion 6 is formed (FIG. 1D).
Due to physical changes within the tissue during the ablation process, the desired thermal lesion 6 illustrated in FIG. 1D is typically difficult to achieve in a single RF application. For example, the concentration of heat adjacent the wires 4 often causes the local tissue to desiccate, thereby reducing its electrical conductivity. As the tissue conductivity decreases, the impedance to current passing from the electrode to the tissue increases so that more voltage must be supplied to the electrodes to affect the surrounding, more distant tissue. The tissue temperature proximate to the electrode may approach 100° C., so that water within the tissue boils to become water vapor. As this desiccation and/or vaporization process continues, the impedance of the local tissue may rise to the point where a therapeutic level of current could no longer pass into the surrounding tissue.
Thus, the rapid fall-off in tissue temperature ultimately limits the volume of tissue that can be therapeutically treated by each of the wire electrodes. As such, depending on the rate of heating and how far the wire electrodes are spaced from each other, ablation devices that have multiple spreading wires may fail to create complete and uniform lesions. While wire electrodes can be repeatedly repositioned to treat additional tissue, the precise movement required for this task is difficult to accomplish.
For these reasons, it would be desirable to provide improvements to ablation devices, such as, e.g., those described in U.S. Pat. No. 5,855,576, so that they could create complete or more uniform lesions.